The invention relates to waveform recorders and in particular to a means and method for triggering a waveform recorder upon the disruption or interruption of a normally continuous signal.
Waveform recorders are general purpose, time domain measuring instruments that take samples of analog waveforms and store the sample values as digital data suitable for analysis by a computer. Waveform recorders are generally suited to measurement of both transient or single-shot waveforms and continuous or repetitive waveforms. Waveform recorders typically acquire a large amount of data over a short period of time to capture a representative sample of a repetitive waveform or to record the occurrence of a single-shot event.
Since the waveform is captured in a digital memory, all digital waveform recorders have some type of pretrigger. Typically, the memory records data continuously in a circular configuration, recording new data over existing data until the trigger stops the recording. An adjustable delay may be used after the trigger to change the time frame of the data captured. For example, with zero delay, the recorder will capture waveform data that occurred before the trigger event, so the cause of the trigger event can be investigated. With a delay equal to the duration that can be stored in the memory, the captured data will start at the trigger event. With longer delays, data at some time after the trigger event can be captured.
One of the most important aspects in designing a digital waveform recorder is to trigger accurately and properly. Converting the data to digital form has made setting the trigger level both accurate and repeatable. The gintroduction of hysteresis trigger controls, which vary the width of the band that the signal must go through before a trigger can occur, has made it possible to trigger reliably on small signals or on large signals with large amounts of noise. However, all triggers have required the occurrence of an event in which the signal crosses the preset trigger level. Thus it has been difficult to record waveforms that are characterized by the non-occurrence of an expected event, where a normally active signal becomes inactive or altered, because there has been no way to automatically trigger on such a non-event.
The area of computer disc testing has made clear the limitations and the disadvantages of conventional triggering techniques. The electromechanical analysis of the head positioning servo system, the read/write electronics and the magnetic material on the disc requires a search for and isolation of infrequent errors and imperfections. An error is characterized by a "drop out", a missing pulse in an otherwise continuous string. In the past, these dropouts were located by testing a checkword at the end of each sector of the disc. An error condition in the checkword was a pretrigger for recording the signal from the defective sector--256 bytes. Then the recorded data was sent to a computer and a pattern recognition routine run to locate the error.
In many other measurement situations a "drop out" trigger would make waveform recording possible or at least greatly simplify the procedure. In industrial power distribution, a device with high in-rush current can cause a temporary drop in line voltage, adversely affecting other devices on the same main, especially computers. It has been difficult or impossible to capture such a disruption of a continuous signal with conventional triggers. However, a drop out trigger could trigger on the drop in line voltage for one cycle, allowing the events surrounding the line voltage drop to be captured and studied. Carrier loss or distortion in communication systems, and sync loss or distortion in video transmission systems could similarly be captured using a waveform recorder equipped with a drop out trigger.
The invention provides means and a method for achieving a "drop out" trigger function. According to a preferred embodiment in accordance to the invention, the drop out trigger is realized by adding a timer circuit to a conventional trigger detection circuit. The timer monitors the elapsed time between trigger events. The conventional trigger detects a parameter periodically satisfied by the continuous signal and resets a timer whenever a trigger event occurs. The alarm count of the timer is set for an interval slightly larger than the periodic interval. Whenever a trigger event occurs, the timer gets reset. If the timer reaches the alarm count, a measurement signal is generated which can be used to stop the measurement and view the waveform or to begin another countdown timer interval.